This invention relates to the problem of impregnating and coating porous webs, and coating but not necessarily impregnating surfaces having little or no porosity, with coating material in aqueous or non-aqueous slurry form, particularly with such material in highly viscous form. The expression "coating material" as used herein should be taken to include both material which simultaneously impregnates a porous web and coats its surface, and material which coats a porous or nonporous web with little or no penetration.
In the formation of many products in flat sheet form it is desirable to be able to apply to a base web a highly viscous material either as a coating or as both an impregnating and a coating material. An example of a product which is both impregnated and coated is a blackout screen formed from a porous web with the same material applied both to impregnate and coat it. An example of a product which is primarily coated with but a small degree of penetration is backed carpet made by coating the back or under side of a needle-punched carpet with a highly viscous material which locks the fiber tufts in place and provides an abrasion-resistant, crush-proof backing with excellent flexibility. In either case, what may start out as a highly irregular surface will end up with a smooth overcoating.
The problem of filling as well as coating with a highly viscous material has been solved in various ways, but these have their limitations. The use of high viscosity foams is well known in the carpet trade, but such foams tend to be pressure sensitive and also to powder on aging.
Treatment by dipping in a bath of coating material and then squeezing off the excess between rolls is another common coating method but it is not acceptable where one surface must remain uncoated, as in carpet manufacture, or where it is desired to hide the texture of the support fabric.
In order to impregnate a porous web and then build up a substantial thickness of coating on its surface or to form a thick coating on a non-porous substrate, it is generally necessary to run the substrate through a succession of at least two separate coating operations. In addition to the high cost of running a web through successive operations, such sequential treatments have serious disadvantages in terms of result as compared to what can be produced in a single operation with the method and apparatus of this invention. During successive coatings blistering may occur when air bubbles are produced in gaps between two strata of coating due to poor adhesion as between a dry first coat and a second coat, particularly if the second coat is a better film former or more elastomeric than the first coat. Flaking and cracking may also result from poor adhesion. Uneven surfaces also result from cockles which occur in papers being coated because of uneven stresses due to variations in moistures across the sheet. In a two stage coating operation, such conditions can be initiated in the first coating pass and magnified in the second coating pass. Thus, it would not only be much more economical to be able to fill and thickly coat a porous web and thickly coat a non-porous surface in one pass through a single coater, but the above described process disadvantage of successive coating would be eliminated.
But there are limitations in how filling and coating porous webs can be accomplished no matter how it is done because the fibers of many desirable base webs, particularly non-woven mats, are held together by thermoplastic fibers and/or heat or solvent-vulnerable binder materials so that such webs cannot stand the application of hot or solvent system materials without disintegrating.
It therefore becomes desirable to be able to impregnate and coat a porous web in one pass; to be able to coat either an impervious substrate or a porous web with a nondestructive coating material, namely, a coating material in aqueous or non-aqueous slurry form having a high viscosity; to be able to achieve, in the case of impregnation, up to 95 percent penetration, and to be able to obtain a coating material add-on by weight of the base web on the order of 350 to 500 percent. And it is particularly important to be able to do all this while at the same time producing on a substrate, either impervious or porous, a surface of such uniform texture and structure that it is free of the numerous defects mentioned above.
The creation of a machine, method and products thereof to accomplish these purposes is the general object of this invention.